1911] Michael: Chaetognatha of the San Diego Begion. 99 



GENERAL DISCUSSION OF THE PROBLEMS OF DISTRIBUTION 



Any interpretation of the data contained in the above tables 

 depends on what we regard as adequate methods of collecting. 

 The numerous methods employed may be distinguished accord- 

 ing to the purpose in collecting. If we desire to study a single 

 species from some morphological, physiological, or other special 

 standpoint, or if we desire to study the number and range of 

 species over extensive areas, we will employ any apparatus 

 adapted to collecting the material. If, however, we desire to 

 study quantitative relations among species, or the abundance 

 of each in any region, we must employ specially constructed 

 apparatus and collect with respect to the conditions affecting 

 this abundance. 



Most single expeditions have endeavored to increase our 

 knowledge of the number and range of species over extensive 

 areas. For instance, the monumental achievements of the ' ' Chal- 

 lenger" expedition consist in having collected and described an 

 enormous number of species representing nearly every class of 

 animals. To determine the general environment of the species 

 obtained from each locality, the hydrographic and meteorological 

 conditions were investigated. The knowledge thus acquired is 

 exceedingly valuable but, owing to the immense region explored, 

 it is necessarily lacking in detail. The methods of collecting 

 employed, while adequate for purposes of preliminary explora- 

 tion, did not obtain sufficient data to permit interpretations 

 respecting the abundance of each species collected. Such inter- 

 pretations depend entirely on more special and detailed methods 

 of collecting. 



These special methods have been mainly developed under the 

 auspices of the "Kiel Commission" by Hensen, Lohmann, 

 Apstein, and other German investigators. They deserve much 

 credit for originating the methods and inventing valuable 

 apparatus for collecting plankton and analyzing the chemical and 

 physical conditions of the sea. They have calculated filtration 

 capacities, and coefficients of correction for a variety of nets, and 

 have materially increased our mathematical and experimental 

 knowledge concerned in quantitative estimates of plankton. Any 



